1. Field of Invention
The present invention relates to a method for recognizing at least one defined pattern modeled by hidden Markov models in a time-variant measurement signal on which at least one disturbing signal is superposed.
2. Description of the Related Art
Pattern recognition is gaining in importance in technical systems, due to increased use of automatization. The patterns are often contained in measurement signals obtained in one way or another from processes that are to be examined. Examples include the analysis of natural language or the examination of executed signatures. Hidden Markov models have proven particularly suitable for the automatic analysis of measurement signals. For example, see the publication by Rabiner, et al, "An Introduction to Hidden Markov Models".
However, a problem in the automatic analysis of measurement signals is that the measurement signals to be examined are often overlaid with variable or occasional disturbing signals or with disturbing signals that are constant in quantity. In the acquisition and analysis of naturally uttered speech, these can be e.g. background noises, breathing noises, machine noises, or also disturbing noises that arise due to the recording medium and the transmission path. For other measurement signals, analogous error sources are possible. In order to be able to find a known pattern within a larger pattern despite the existing difficulties, an approach with special hidden Markov models has been proposed in the publication by Rose et al., "A Hidden Markov Model Based Keyboard Recognition System". A specific model is hereby introduced (termed a garbage or background model) that models background noise, including other speech. This specific model must in all cases be trained with corresponding noises or, respectively, speech. This means that only those noises or, respectively, disturbances that are present in the training material can be taken into account when recognizing a pattern. In each case, the modeling of this model has a large influence on the overall probability of a key word, or, of any pattern to be recognized. A further disadvantage is that this background model also recognizes speech sounds, or, respectively, pattern portions in other types of measurement signal, which actually belong to a pattern or, respectively, a keyword to be recognized. The additional problem of a suitable weighting of the model in order to avoid an excessively low detection rate thereby results, as discussed in the Rose publication. Further possibilities for taking into account disturbing portions in the analysis of measurement signals, using hidden Markov models, are not known from the prior art.